This invention relates to an induction electric motor, and more specifically to a means and a method which may be readily used with the motor so as to result in operation of the motor at a speed slower than its normal of operational speed.
In such applications as in a ceiling-mounted fan or the like, a single or multi-speed motor is typically provided for driving the fan. However, in many instances, it would be desirable to provide another speed of operation for the fan motor even slower than its normal low speed mode of operation. Typically, this is accomplished by replacing a single speed motor with a multi-speed motor, or by replacing a two-speed motor with a three-speed motor. These multi-speed motors oftentimes have different sets of main windings each having different number of poles in each set of windings which may be energized for high, low, or intermediate speed operation. For example, a two-speed motor may be provided having a low speed set of windings including a four poles and a high speed set of windings including two poles. Typically, a selector switch is connected to the lead wires to the motor windings for selective energization for either the high speed or low speed poles of the motor. Of course, it will be appreciated that by providing additional sets of windings or by selective energization of the various coils in the windings, many more speeds of operation of the motor may result. However, it is usually more expensive to provide an additional set of windings in the motor as these additional windings require more materials to construct, take up more space within the motor, and are more difficult to insert in the stator core of the motor.
In a multi-winding motor, the motor generally exhibits its higher starting torque capability when started in its high speed mode of operation. In certain motor applications requiring good starting torque, a multi-speed motor may not have sufficient torque for start up purposes unless it is started in its higher speed modes of operation.
Alternatively, another method of speed control is to place an inductance in series with the main windings of the motor so that with the load current flowing through the main windings and the inductance the inductance will cause the motor to operate at a slower speed. Such a voltage reduction speed control is shown in U.S. Pat. No. 850,203. However, while such a voltage controlling speed control may be relatively simple, and may be "added on" to the motor, it results in significant heat losses. Also, such a voltage reduction speed controller means that when the inductance is connected to the main windings, the motor may not have sufficient starting torque. As is disclosed above-noted U.S. Pat. No. 850,203, a centrifugally responsive switch may be provided to disconnect the resistance from the winding during start-up and to reconnect the resistance to the windings only after the motor has accelerated to a predetermined speed. However, in many motor applications, there may not be sufficient space within the motor to use the centrifugal switch approach and the centrifugal switch adds to the complexity and the cost of the motor.
Among the several objects and features of the present invention may be noted the provision of speed control means which may readily be incorporated with a motor so as to result in the selected operation of the motor at a slower speed;
The provision of such a speed control means which is of relatively compact size;
The provision of such a speed control means switch may be "added on" to the motor wiring externally of the motor;
The provision of such a means and method of speed control which does not generate an appreciable amount of heat and thus which is energy efficient;
The provision of such speed control means which, in one embodiment thereof, does not adversely affect the starting torque characteristics of the motor even when the motor is set for slow speed operation; and
The provision of such speed control means which is of low cost construction and which is reliable in operation.
Other objects of this invention will be in part apparent and in part pointed out hereinafter.